Contact Lens Vertex Calculator
Enter your spectacle lens power and the vertex distance to calculate the equivalent contact lens power, since moving a lens closer to the eye changes its effective power.
Why vertex distance matters
A lens’s optical power depends on where it sits relative to the eye. Spectacle lenses sit a small distance in front of the eye (the vertex distance), while contact lenses sit directly on the cornea, so an equivalent power correction is needed to switch between them, especially for stronger prescriptions.
The formula
The vertex distance conversion formula is Fc = Fs ÷ (1 − d × Fs), where Fs is the spectacle lens power in diopters, d is the vertex distance in meters, and Fc is the resulting contact lens power.
This correction becomes more significant for prescriptions stronger than about ±4.00 D; for milder prescriptions the difference is often clinically negligible. This calculator provides a general optical estimate for educational purposes and does not replace an eye care professional’s actual contact lens fitting and prescription.